Bugfix: mf.optional for list types

This MR enables (or fixes mf.optional for) list types.

Description

Miniflask did not allow to specify empty CLI-lists previously.
Using mf.optional this is now possible.

Not specifying the variable in CLI results in an empty list.

Check all before creating this PR:

• Documentation adapted
• unit tests adapted / created

Example Usage

def register(mf):
    mf.register_defaults({
       "variablelist": mf.optional([int])
    })